1. Field of the Invention
The present invention relates to a guest-host liquid crystal display device. Particularly, the present invention relates to the composition of a nematic liquid crystal material as a host in which a dichroic dye serving as a guest is dissolved.
2. Description of the Related Art
A guest-host liquid crystal display device uses as an electro-optical material a guest-host liquid crystal in which a dichroic dye is dissolved in a nematic liquid crystal serving as a host. The alignment of the dichroic dye is controlled together with molecules of the nematic liquid crystal by the function of an electric field. The transmitted light of the guest-host liquid crystal display device can be controlled by the anisotropy of light absorption possessed by the dichroic dye. The types of gust-host liquid crystal display devices include a transmission type and a reflection type. In both cases, a black dichroic dye is used for white-black display. This display can be combined with a micro-color filter to obtain a full color image display. Therefore, as the dichroic dye, a black dye suitable for black display is generally selected. The guest-host liquid crystal is previously aligned horizontally or vertically in a cell, and switched to horizontal alignment or vertical alignment by turning a voltage on or off. When the liquid crystal has previously been aligned horizontally, the guest-host liquid crystal display device causes black display in the off state of the voltage, and is switched to the vertical alignment by turning the voltage on to cause white display. Conversely, when the liquid crystal has previously been aligned vertically, the absorption of the dichroic dye is decreased to form white display in the off state of the voltage. In the on state of the voltage, since the liquid crystal molecules and the dichroic dye are transferred to the horizontal alignment, absorption is increased to form black display.
FIG. 8 shows the applied voltage/transmittance characteristics of guest-host liquid crystal display devices. In FIG. 8, the characteristics of a horizontal alignment cell are shown by black dots, and the characteristics of a vertical alignment cell are shown by black squares. In the horizontal alignment cell, the transmittance slowly increases as the applied voltage increases, and a high voltage must be applied for obtaining a sufficient level of white display. On the other hand, in the vertical alignment cell, the white level is high without application of a voltage, and a sufficient level of black display can be obtained by applying a relatively low voltage. Therefore, the vertical alignment cell is superior to the horizontal alignment cell for obtaining a high contrast by low-voltage driving. In the vertical alignment, the liquid crystal molecules has previously been aligned vertically without application of a voltage, and it is thus necessary to use a nematic liquid crystal material exhibiting the behavior that its molecules are aligned horizontally with application of a voltage. The liquid crystal material exhibiting such a behavior is required to have negative dielectric anisotropy as a basic requirement. In addition, the driving voltage decreases as the value of the negative dielectric anisotropy increases in the negative direction. However, it is difficult to control a liquid crystal to vertical alignment, and there are few liquid crystals having a large absolute value of negative dielectric anisotropy. Therefore, the vertical alignment cell is hardly brought into practical use.
In consideration of the problems of conventional techniques, an object of the present invention is to provide a guest-host liquid crystal display device using a host liquid crystal which can easily be aligned vertically and which has a large absolute value of negative dielectric anisotropy.
In order to achieve the object, the following means is provided. A guest-host liquid crystal display device of the present invention basically comprises a pair of substrates which are bonded with a predetermined gap therebetween, a guest-host liquid crystal containing a dichroic dye and held in the gap, and an electrode formed on each of the substrates, for applying a voltage to the guest-host liquid crystal. The guest-host liquid crystal display device is characterized in that the guest-host liquid crystal mainly comprises a nematic liquid crystal composition having a value of dielectric anisotropy within the range of xe2x88x922 to xe2x88x9210, and has previously been aligned vertically with respect to the upper and lower substrates. The guest-host liquid crystal preferably contains a black dichroic dye, and is switched from an achromatic transmission state to an absorption state when applying a voltage, and a color filter is provided on one of the substrates to make the transmission state chromatic. Preferably, one of the substrates is transparent and is positioned on the incident side, and the other substrate is positioned on the reflection side and has a reflecting layer and a quarter wavelength plate layer which are formed in turn thereon.
In accordance with the present invention, the guest-host liquid crystal display device uses as an electro-optical material a liquid crystal composition having negative dielectric anisotropy, for example, a guest-host liquid crystal containing the black dichroic dye dissolved therein. The value of dielectric anisotropy is set within the range of xe2x88x922 to xe2x88x9210. The guest-host liquid crystal has previously been aligned vertically in a cell to secure a sufficiently high level of white display without application of a voltage. With application of a voltage, the dichroic dye is aligned horizontally together with the liquid crystal molecules having negative dielectric anisotropy to realize a sufficiently low black level. This can provide a display with a good contrast. The guest-host liquid crystal display of the prevent invention can secure a threshold voltage Vth of about 2 V. Namely, the white display and black display are switched by turning on and off the applied voltage with the threshold voltage Vth of about 2 V as a boundary. It is, of course, possible to realize an intermediate tone display between both displays. In a guest-host liquid crystal display device using thin film transistors for switching and driving liquid crystal pixels, the voltage resistance of the thin film transistors is about 5 V, and thus low-voltage driving is preferred. In this case, the gust-host liquid crystal display device of the present invention having Vth of about 2 V can sufficiently be used for driving at 5 V. If the value of dielectric anisotropy xcex94xcex5 is brought closer to zero from xe2x88x922, Vth is undesirably increased to 2 V or more. Although Vth decreases as the value of dielectric anisotropy xcex94xcex5 increases in the negative direction, there is no liquid crystal having a value xcex94xcex5 of dielectric anisotropy larger than xe2x88x9210 in the negative direction within a practical range. Therefore, the limit of the value xcex94xcex5 is about xe2x88x9210 from the viewpoint of limits of materials.